Method and device for drying a moving web material

ABSTRACT

A method and device for drying a moving web including a first infrared radiator arranged on a first side of the web and a second infrared radiator arranged on a second side of the web at least partially opposite to the first infrared radiator and proximate to the first infrared radiator. The wavelength of the maximum intensity of the radiation generated by the first infrared radiator is shorter than the wavelength of the maximum intensity of the radiation generated by the second infrared radiator. The power density of the first infrared radiator is from about 450 kW per sq.m to about 700 kW per sq.m and the emitter temperature of the first infrared radiator is from about 2000° C. to about 2800° C. The second infrared radiator includes a surface layer made of a metal, metal alloy or ceramic material whose emissivity is substantially equal to or higher than about 0.6.

FIELD OF THE INVENTION

The invention concerns a method for drying a moving web material, inwhich method infrared radiation is directed at the material to be driedand in which method the moving web material is passed through theradiation one of an infrared radiator while the web material to be driedabsorbs radiation into itself, in which method the radiation produced byat least one first infrared radiator and the radiation produced by atleast one second infrared radiator are applied to the moving webmaterial to be dried, said radiators being fitted in the vicinity of oneanother, and the wavelength of the maximum intensity of the radiation ofsaid first infrared radiator being shorter than the wavelength of themaximum intensity of the radiation of said second infrared radiator, inwhich case, in the drying process, the spectrum of the overall radiationis optimal in view of the absorption spectrum of the material to bedried, and in which method the first infrared radiator is placed at oneside of the web material and the second infrared radiator at theopposite side.

The invention also concerns a device for drying a moving web material,which device is fitted to direct infrared radiation at the moving webmaterial to be dried, and which device comprises at least one firstinfrared radiator and at least one second infrared radiator, which arefitted at the vicinity of one another, and the wavelength of the maximumintensity of the radiation of the first infrared radiator being shorterthan the wavelength of the maximum intensity of the radiation of saidsecond radiator, and in which device the first infrared radiator isplaced at one side of the web material and the second infrared radiatorat the opposite side.

BACKGROUND OF THE INVENTION

In paper and textile industries and also in other fields of industry, amoving web material is dried. In the production and finishing of paper,there are a number of stages at which drying has to be carried out bymeans of a method not contacting the web, for example by drying by meansof radiation.

The infrared radiator devices currently used for drying of a webmaterial consist of high-temperature quartz-tube radiators or ofgas-operated medium-wave radiators. The wavelength range of ahigh-temperature short-wave radiator is substantially 0.5 . . . 5.0 μm,while the peak is at about 1.2 μm. When a thin web is dried, theshort-wave radiation penetrates through the web, because the absorptioncoefficient of the material is, as a rule, poor in the wavelength rangebetween 0.5 μm and 2.0 μm, as the absorption peak is in a rangesubstantially higher than 2 μm. Thus, the emission peak of the radiatorand the absorption peak of the web material do not coincide. However,with a high-temperature short-wave radiator, a high power density perunit of area is achieved. The power density may be up to 450 kW per sqm, in which case the radiation energy absorbed into the web is higherthan 130 kW per sq m. Power densities of said order are required in anattempt to obtain quick drying, which is again necessary, for example,in a process of coating of paper.

The wavelength range of medium-wave infrared radiators is substantially1.5 μm . . . 6.0 μm. The wavelength corresponding to the maximumintensity is placed approximately between 2.0 μm and 3.0 μm. One of thepoints of absorption maximum of the water to be evaporated is situatedwithin said interval. At said interval, the absorptivity of cellulosicfibres is also good. Out of the reasons mentioned above, the radiationefficiency of the radiation of a medium-wave radiator is high, about40-60%, whereas the corresponding efficiency with short-wave infraredradiators, i.e. with a high-temperature radiator, is about 30-35% whendrying of thin web materials is concerned. When the thickness of thematerial is increased, the efficiency of absorption becomes higherespecially for the short-wave radiators.

The maximum power density attainable with medium-wave infrared radiatorsis 60 . . . 75 kW per sq m when a one-sided source of radiation is used,and 120 . . . 150 kW per sq m when a two-sided source of radiation isused.

A dryer composed of an infrared radiator device, i.e. an IR-dryer,consists of a radiation face, which is placed as close to the face to bedried as possible. In the prior-art devices, the radiation face isenclosed in a box, and the box is fixed in a suitable location on theframe constructions of the- process equipment either stationarily or asprovided with a displacing mechanism. Further, in said dryers, the useof a backup reflector is known, which reflects the radiation that haspassed through the material to be dried and thereby intensifies theprocess of drying.

From the prior art, a number of different IR-dryers used for drying of amoving web or web material are known. The operation of these dryers isbased on the ability of pieces to emit electromagnetic radiation, whichis specific of the temperature of the piece. It is a second featurecharacteristic of radiation that, instead of one wave-length, theradiator emits several wavelengths, whereby an emission spectrumspecific of the radiator is formed. Further, in accordance with the lawsof physics, it is characteristic of radiation that, when the temperatureof the radiating piece rises, the transfer of radiation heat to thetarget material is increased in proportion to the difference between thefourth powers of the temperatures of the pieces.

However, the temperature of the radiator does not alone determine howmuch radiation can be absorbed into the material to be dried. Thetemperature, moisture, thickness, material, surface roughness, andbrightness of the piece to be dried determine an absorption coefficient,which indicates what a proportion of the radiation arriving on the faceof the piece to be dried is absorbed into the material. However, as arule, the absorption coefficient is a function of the wavelength, sothat in a short-wave range the absorption coefficient of a thin materialis inferior to that in a medium-wave or long-wave range.

IR-radiation sources operating in the short-wave infrared range areconsidered radiators which emit a radiation whose wavelength of maximumintensity of radiation is in the wavelength range of 0.76 . . . 2.00 μm.IR-radiation sources operating in the medium-wave infrared range areconsidered radiators which emit a radiation whose wavelength of maximumintensity is in the wavelength range of 2.00 . . . 4.00 μm.

The correspondence with temperature is obtained by means of Wien'sdisplacement law from the formula

    λ.sub.maximum ×T=2.8978·10.sup.-3 (mK)

The temperature range of a short-wave radiator is obtained as 3540° C. .. . 1176° C., and that of a medium-wave radiator as 1176° C. . . . 450°C.

The IR-dryers operating in the short-wave range are currently almostexclusively electrically operated. In them, usually a tungsten filamentplaced in a quartz tube is made to glow by means of electric current.The maximum emitter temperature of the glowing filament is usually about2200° C., in which case the wavelength corresponding to the maximumintensity of radiation is about 1.2 μm.

In the prior-art short-wave infrared radiators, the lamps are, as arule, arranged in heating modules of 3 . . . 12 lamps. The modules areattached side by side, and a drying zone extending across the web isobtained. The lamps are usually spaced so that the power density of thedryer per unit of area varies in a range of 100 . . . 450 kW per sq m.

The dryers operating in the medium-wave IR range are either electricallyoperated or gas-operated. In electric devices, filaments are made toglow by means of electric current either in a quartz tube or behind aceramic tile or a tile made of quartz. In the former case, the spiralfilament operates directly as the emitter, whereas in the latter casethe heat is transferred first into the tile, after which the tileoperates as the emitter. The tile may also be partly penetrable byradiation. In gas-operated systems, a usually ceramic radiator is madeto glow by means of a flame, which radiator starts glowing and thusoperates as the emitter. Radiation is partly also emitted directly fromthe flame. As was stated above, the wavelength of maximum intensity ofmedium-wave infrared radiators is 2.00 . . . 3.00 μm, the correspondingtemperature of the radiator being, as was stated above, in the range of1176° C. . . . 690° C. With medium-wave infrared radiators, the maximumpower density varies, depending on the method and the temperature,substantially in a range of 40 . . . 100 kW per sq m.

Adverse aspects of short-wave infrared radiators include-poor radiationefficiency in the shorter wavelength range of the radiator influencingthe overall efficiency, expensive electric control system, high cost forelectricity and ventilation systems.

Adverse aspects of medium-wave infrared radiators include low powerdensity per unit area when quick drying is aimed at, poor adjustability,slow heating and cooling, relatively high cost of electrical system andelectricity in the case of electric infrared radiators. For gas operatedsystems the high cost for the gas feed system and the risk of explosionfrom handling of explosive gases can be mentioned.

The difficulties to use the cooling exhaust air or the exhaust gases foran efficient improvement of the drying process is common for both gas-and electrical medium wave dryers.

Thus, it can be considered that a major drawback of the prior artinfrared heaters, ie. IR-dryers, consisting of short wave infraredradiators is poor efficiency because of the low absorption coefficientof the material to be dried in the shorter wave length range of theradiator.

When the IR-dryer consists of medium wave infrared radiators, aparticular drawback can be considered to be the low power density andstill the need for a relatively expensive electrical and ventilationsystem, poor controllability because of the slow heating and cooling ofthe medium-wave radiators and the difficulties to efficiently use theexhaust air or gases in the drying process.

In the EP Patent 288,524, a method is described for drying a moving webmaterial.

In the method, infrared radiation is directed at the material to bedried, and the moving web material is passed through the radiation zoneof the infrared radiator while the web material to be dried absorbsradiation into itself. In the method, the radiation produced by at leastone first infrared radiator and the radiation produced by at least onesecond infrared radiator are directed at the moving web material to bedried, said radiators being fitted in the vicinity of one another. Inthis connection, the wavelength of the maximum intensity of theradiation of the first infrared radiator is shorter than the wavelengthof the maximum intensity of the radiation of the second infraredradiator, in which case, in the drying process, the spectrum of theoverall radiation is optimal in view of the absorption spectrum of thematerial to be dried. The maximum intensity of the radiation of thefirst infrared radiator occurs in the wavelength range of the radiation0.76 μm<λ_(maximum) <2.00 μm, and the maximum intensity of the radiationof the second radiator is in the wavelength range 2.00 μm<λ_(maximum)<4.00 μm. The radiators can be fitted at the same side of the moving webmaterial, or they can be fitted so that the first radiator is placed atone side of the web material and the second radiator at the oppositeside.

By means of the method and the device in accordance with the EP Patent288,524, a spectrum is obtained that is favourable in view of thedrying. Then, an efficiency of radiation is achieved that is at leastabout 5% better than with the prior-art solutions of equipment.

From the prior art, it is known to provide the second radiator, placedat the opposite side of the web material to be dried, with a surfacelayer which in the short wave 0,5-2,0 μm spectra mainly reflects butpartly also absorbs the radiation of the first infrared radiator thatpasses through the material web so that the temperature of the secondinfrared radiator rises to several hundreds of Celsius degrees. When atypical white ceramic material is used as the surface material, thetemperature may rise to a value of an order of 500 . . . 700° C. for lowgrammage webs for example paper webs with grammages less than 110 g/m².A temperature of 500 . . . 700° C. is not yet sufficient as the surfacetemperature of the second infrared radiator, while its power density isa function of its temperature level in Kelvin degree in fourth power,but additional electric energy can be fed into the surface layer of thesecond infrared radiator according to EP Patent 288,524, whereby thesurface temperature can be raised further to a temperature of 800 . . .1050° C.

Thus, the backup radiator described above is a device that receives theheat radiation passing through the web and uses this heat for heatingthe surface layer of the device. The backup radiator is a medium-waveradiator. The backup radiator is used together with a short-waveinfrared radiator. Together, these two devices produce a good dryingresult and efficiency.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide an improvement overthe method and the device described in the EP Patent 288,524 for dryinga moving web material. A specific object of the present invention is toprovide a method and a device wherein it is possible to avoid the supplyof additional electric energy to the surface layer of the secondradiator.

The objectives of the invention are achieved by means of a method whichis characterized in that

(a) as the first infrared radiator, a radiator is used whose powerdensity is 450 . . . 700 kW per sq m and whose emitter temperature is2000 . . . 2800° C.,

(b) as the web material to be dried, a web is used whose transmissivityis substantially higher than, or equal to, 0.18 for short wave infraredradiation 0.5-2.0 μm,

(c) as the second infrared radiator, a radiator is used whose surfacelayer is made of such a metal, metal alloy or ceramic material whoseemissivity is substantially higher than, or equal to, 0.6, within thetotal wavelength range of 0.5-2.0 μm.

in which case, of the power density of the first infrared radiator, sucha percentage proportion passes through the web as is sufficient to becapable of heating the surface layer of the second infrared radiator toa temperature of substantially at least 800° C.

On the other hand, the device in accordance with the invention ischaracterized in that the power density of the first infrared radiatoris 450 . . . 700 kW per sq m and the temperature 2000 . . . 2800° C.,and the surface layer of the second infrared radiator is made of ametal, metal alloy or ceramic material whose emissivity is substantiallyhigher than 0.6 within the total wavelength range of 0.5-2.0 μm.

The device and the method in accordance with the present invention areparticularly well suitable for thin web grades, which have atransmissivity τ equal or higher than 0.18 for short wave radiation forexample corresponding to grammages equal or less than 110 g/m² forordinary paper webs. As the first radiator, a radiator is used whosepower density is 450 . . . 700 kW per sq m and whose temperature is 2000. . . 2800° C. As the second radiator, a radiator is used whose surfacelayer is made of a metal, metal alloy or ceramic material whoseemissivity is substantially higher than 0.6 within the total wavelengthrange of 0.5-2.0 μm. In such a case, of the power density of the firstradiator, such a percentage proportion of the energy passes through theweb as is sufficient to heat the surface layer of the second radiator toa temperature of substantially at least 800° C.

In a preferred embodiment of the invention, the power density of thefirst radiator is chosen at a value of 530 . . . 650 kW per sq m, andthe temperature with the maximum power density at the value 2100 . . .2600° C., and the emissivity of the surface layer of the second radiatoris chosen at a value of 0,65-0,9 within the total wavelength range of0.5-2.0 μm.

In a preferred embodiment of the invention, the surface layer is formedof a metal alloy which contains 10 . . . 26%-wt. (per cent by weight) ofchromium, 0 . . . 84%-wt. of iron, and 0 . . . 81%-wt. of nickel and0-25%-wt. of aluminium. A metal alloy is particularly favourable whichcontains chromium, >20%-wt. of iron and alternatively nickel oraluminium or a metal alloy of chromium and nickel.

In a preferred embodiment of the invention, ceramic material has beenchosen from the group of carbides, nitrides and suicides.

In an another preferred embodiment of the invention, ceramic material isa ceramic base, preferably an aluminium oxide, zirconium oxide, glassceramic or quartz material, coated with a carbide, nitride, silicide, ametal or a metal alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to somepreferred embodiments of the invention illustrated in the figures in theaccompanying drawings, the invention being, however, not supposed to beconfined to said embodiments alone.

FIG. 1 is a schematic side view illustrating a prior-art method fordrying a web material.

FIG. 2 is a schematic side view illustrating the basic principle of themethod in accordance with the present invention.

FIG. 3 is a perspective view of a first embodiment of a radiator traywhich is a part of the second radiator in FIG. 2.

FIG. 4 is a planar view from above of the radiator tray shown in FIG. 3.

FIG. 5 is a view from above in FIG. 4.

FIG. 6 is a view from the left in FIG. 4.

FIG. 7 is a view corresponding to FIG. 6, but with the flanged sheet inthe left edge dismounted.

FIG. 8 is a partially sectioned view according to the line VIII--VIII inFIG. 4.

FIG. 9 is an enlarged view of a part A of FIG. 8.

FIG. 10 is a perspective view of an alternative embodiment of a radiatortray which is a part of the second radiator in FIG. 2.

FIG. 11 is a planar view from above of the radiator tray shown in FIG.10.

FIG. 12 is a view from above in FIG. 11.

FIG. 13 is a view from the left in FIG. 11.

FIG. 14 is a view corresponding to FIG. 13, but with the flanged sheetin the left edge dismounted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the prior-art solution shown in FIG. 1, the web material to be driedis denoted with the letter P. The web material passes over the rolls 13and 14, and the running direction of the web material P is denoted withthe arrow A. The first infrared radiator 11 is placed at one side of theweb P, and similarly the second infrared radiator 12 is placed at theopposite side of the web P. The infrared radiator 11 and the infraredradiator 12, respectively, may consist of one or several separateradiators. When solutions known from the prior art are used as thesurface layer in the second radiator, the radiation of the firstinfrared radiator 11 that passes through the web P can heat the surfacelayer of the second radiator 12, at the maximum, to a temperature ofabout 500 . . . 700° C.

In FIG. 2, the surface layer in accordance with the present invention isdenoted with the reference numeral 15. The power density of the firstinfrared radiator 11 is chosen as 450 . . . 700 kW per sq m, and thetemperature is chosen as 2000 . . . 2800° C. As the surface layer 15 ofthe second radiator 12, a metal, metal alloy or a ceramic material isused, whose emissivity is substantially higher than, or equal to, 0.6within the total wavelength range of 0.5-2.0 μm. When a web materialwith equal or higher transmissivity τ than 0.18 for short wave infraredradiation which for example with ordinary paper webs correspond togrammages substantially equal or less than 110 g/m² is used, such apercentage proportion of the intensity of the first radiator 11 passesthrough the web P as is sufficient to heat the surface layer of thesecond radiator 12 substantially at least to a temperature of 800° C.

Advantageously, the surface layer 15 contains 10 . . . 26%-wt. ofchromium, 0 . . . 84%-wt. of iron, and 0 . . . 81%-wt. of nickel,0-25%-wt of aluminium. In a preferred embodiment, the surface layer 15contains a metal alloy with chromium, >20%-wt. of iron and alternativelynickel or aluminium or a metal alloy of nickel and chromium.

The second radiator 12 in FIG. 2 have a frame on which box-shapedradiator trays according to FIGS. 3-9 are mounted.

The radiator tray according to FIGS. 3-9 is as a whole marked with 20.It comprises a with heat insulation 22 of ceramic fibres filled radiatorsheet box 23 together with radiator surface material 24, in one orseveral parts, building up the surface layer 15 in FIG. 2.

A radiator surface material or part 24 according to the invention isshown from a side view in FIG. 8. As can be seen from FIG. 8 and FIG. 3is this part bended showing longitudinal waves with tops 25 and grooves26, in which row-vise are arranged holes 27 for mounting of bolts 28with a head 29 and free ends 30 with lock pins 31.

As can be seen from FIG. 9 is the outmost situtated longitudinal row ofholes situated in an eccentric manner to press the outmost free waveeffectively down. In this way the design will prevent the mentionedoutmost waves from bending upwards forming an obstacle for the passingweb or other parts.

According to the design the bolts 28 can be surrounded by distance pipes32 to secure a defined thickness of the total radiator tray 20.

The radiator tray frame can be comprised by sections in which case twoon the opposite side situated flanged sheets 33 are mounted to lay uponthe radiator surface material parts and lock them up in the edges.

An alternative embodiment of a radiator tray 20a according to theinvention is shown in FIGS. 10-14.

The second radiator 12 in FIG. 2 have a frame on which box-shapedradiator trays according to FIGS. 10-14 are mounted.

The radiator tray according to FIGS. 10-14 is as a whole marked with20a. It comprises a with heat insulation 22 of ceramic fibres filledradiator sheet box 23 together with radiator surface matterial 24a inone or several parts building up the surface layer 15 in FIG. 2.

The alternative embodiment can preferably be used if the radiatorsurface material 24a of ceramic material, metal or an metal alloyaccording to the invention have such a mechanical stability over 800° C.that the flanged sheets 33 on both sides are capable to keep theradiator surface material in a fixed position over its total surface.

Above, just the solution of principle of the invention has beendescribed, and it is obvious to a person skilled in the art thatnumerous modifications can be made to said solution within the scope ofthe inventive idea defined in the accompanying patent claims.

We claim:
 1. A method for drying a moving web having a transmissivitysubstantially equal to or higher than about 0.18 for short wave infraredradiation having a wavelength from about 0.5 μm to about 2.0 μm,comprising the steps of:arranging a first infrared radiator on a firstside of the web, said first infrared radiator having a power densityfrom about 450 kW per sq.m to about 700 kW per sq.m and an emittertemperature from about 2000° C. to about 2800° C., arranging a secondinfrared radiator on a second side of the web at least partiallyopposite to said first infrared radiator and proximate to said firstinfrared radiator, said second infrared radiator including a surfacelayer made of a metal, metal alloy or ceramic material whose emissivityis substantially equal to or higher than about 0.6 within the totalwavelength range of from about 0.5 μm to about 2.0 μm, directinginfrared radiation from said first and second infrared radiators at theweb by passing the web between said first and second infrared radiatorssuch that the web absorbs radiation and is dried, operating said firstinfrared radiator and said second infrared radiator such that thewavelength of the maximum intensity of the radiation generated by saidfirst infrared radiator is shorter than the wavelength of the maximumintensity of the radiation generated by said second infrared radiator,and selecting the power density and emitter temperature of said firstinfrared radiator such that a portion of the power density of said firstinfrared radiator passes through the web and is effective to heat saidsurface layer of said second infrared radiator to a temperature of atleast about 800° C.
 2. The method of claim 1, further comprising thestep of:utilizing a paper web having a grammage substantially equal toor less than about 110 grams per sq.m as the web.
 3. The method of claim1, wherein the power density of said first infrared radiator is selectedin the range of from about 530 kW per sq m to about 650 kW per sq m andthe emitter temperature of said first infrared radiator is selected inthe range of from about 2100° C. to about 2600° C.
 4. The method ofclaim 1, further comprising the step of:selecting the metal, metal alloyor ceramic material of said second infrared radiator such that it has anemissivity from about 0.65 to about 0.9 within the total wavelengthrange of about 0.5 μm to about 2.0 μm.
 5. The method of claim 1, whereinsaid surface layer of said second infrared radiator is made of a metalalloy containing chromium, aluminum, nickel and iron.
 6. The method ofclaim 1, wherein said surface layer of said second infrared radiatorcontains 10%-26% of chromium by weight, 0-84% of iron by weight, 0-81%of nickel by weight, and 0-25% of aluminum by weight.
 7. The method ofclaim 1, wherein said surface layer of said second infrared radiator ismade of a metal alloy including chromium, more than 20% of iron byweight, and nickel, aluminum or a metal alloy of chromium and nickel. 8.The method of claim 1, wherein said surface layer of said secondinfrared radiator is made of a ceramic material selected from a groupconsisting of carbides, nitrides and silicates.
 9. The method of claim1, wherein said surface layer of said second infrared radiator is madeof a ceramic material comprising a ceramic base selected from a groupconsisting of aluminum oxide, zirconium oxide, glass ceramic and quartzmaterial, and a coating selected from a group consisting of a carbide,nitride, silicate, a metal and a metal alloy.
 10. A device for drying amoving web, comprisinga first infrared radiator arranged on a first sideof the web and having a power density from about 450 kW per sq.m toabout 700 kW per sq.m and an emitter temperature from about 2000° C. toabout 2800° C., and a second infrared radiator arranged on a second sideof the web at least partially opposite to said first infrared radiatorand proximate to said first infrared radiator, said first infraredradiator and said second infrared radiator being structured and arrangedsuch that the wavelength of the maximum intensity of the radiationgenerated by said first infrared radiator is shorter than the wavelengthof the maximum intensity of the radiation generated by said secondinfrared radiator, said second infrared radiator comprising a surfacelayer made of a metal, metal alloy or ceramic material whose emissivityis substantially equal to or higher than about 0.6 within the totalwavelength range of from about 0.5 μm to about 2.0 μm.
 11. The device ofclaim 10, wherein the power density of said first infrared radiator isin the range of from about 530 kW per sq m to about 650 kW per sq m andthe emitter temperature of said first infrared radiator is in the rangeof from about 2100° C. to about 2600° C.
 12. The device of claim 10,wherein said surface layer of said second infrared radiator has anemissivity from about 0.65 to about 0.9 within the total wavelengthrange of about 0.5 μm to about 2.0 μm.
 13. The device of claim 10,wherein said surface layer of said second infrared radiator is made of ametal alloy containing chromium, aluminum, nickel and iron.
 14. Thedevice of claim 10, wherein said surface layer of said second infraredradiator contains 10%-26% of chromium by weight, 0-84% of iron byweight, 0-81% of nickel by weight, and 0-25% of aluminum by weight. 15.The device of claim 10, wherein said surface layer of said secondinfrared radiator is made of a metal alloy including chromium, more than20% of iron by weight, and nickel, aluminum or a metal alloy of chromiumand nickel.
 16. The device of claim 10, wherein said surface layer ofsaid second infrared radiator is made of a ceramic material selectedfrom a group consisting of carbides, nitrides and silicates.
 17. Thedevice of claim 10, wherein said surface layer of said second infraredradiator is made of a ceramic material comprising a ceramic baseselected from a group consisting of aluminum oxide, zirconium oxide,glass ceramic and quartz material, and a coating selected from a groupconsisting of a carbide, nitride, silicate, a metal and a metal alloy.18. The device of claim 10, wherein said surface layer of said secondinfrared radiator includes ridges and grooves to provide said surfacelayer with a wavy contour.
 19. The device of claim 10, wherein saidsecond infrared radiator comprisesa box having first and second sides,said surface layer having apertures and being arranged on said firstside of said box, a layer of heat insulation arranged in said box, saidlayer of heat insulation having holes therethrough, and bolts arrangedto extend through said apertures in said surface layer, said holes insaid heat insulation and holes in said second side of said box forsecurely retaining said surface layer in connection with said box. 20.The device of claim 19, wherein said apertures in said surface layer arearranged in longitudinal rows such that outermost longitudinal rows ofsaid holes are situated in an eccentric manner.